Novel protein-disulfide isomerases from the early-diverging protist Giardia lamblia

Protein-disulfide isomerase is essential for formation and reshuffling of disulfide bonds during nascent protein folding in the endoplasmic reticulum. The two thioredoxin-like active sites catalyze a variety of thiol-disulfide exchange reactions. We have characterized three novel protein-disulfide isomerases from the primitive eukaryote Giardia lamblia. Unlike other protein-disulfide isomerases, the giardial enzymes have only one active site. The active-site sequence motif in the giardial proteins (CGHC) is characteristic of eukaryotic protein-disulfide isomerases, and not other members of the thioredoxin superfamily that have one active site, such as thioredoxin and Dsb proteins from Gram-negative bacteria. The three giardial proteins have very different amino acid sequences and molecular masses (26, 50, and 13 kDa). All three enzymes were capable of rearranging disulfide bonds, and giardial protein-disulfide isomerase-2 also displayed oxidant and reductant activities. Surprisingly, the three giardial proteins also had Ca(2+)-dependent transglutaminase activity. This is the first report of protein-disulfide isomerases with a single active site that have diverse roles in protein cross-linking. This study may provide clues to the evolution of key functions of the endoplasmic reticulum in eukaryotic cells, protein disulfide formation, and isomerization.

Tellurite-mediated thiol oxidation in Escherichia coli

The oxyanion of tellurium, tellurite (TeO3(2-)), is toxic to most micro-organisms, particularly gram-negative bacteria. The mechanism of tellurite toxicity is presently unknown. Many heavy metals and oxyanions, including tellurite, interact with reduced thiols (RSH). To determine if tellurite interaction with RSH groups is involved in the toxicity mechanism, the RSH content of Escherichia coli cultures was assayed. After exposure to tellurite, cells were harvested and lysed in the presence of the RSH-specific reagent 5,5'-dithiobis(2-nitrobenzoic acid). Upon exposure of tellurite-susceptible cells to TeO3(2-), the RSH content decreased markedly. Resistance to potassium tellurite (Te(r)) in gram-negative bacteria is encoded by plasmids of incompatibility groups IncFI, IncP alpha, IncHI2, IncHI3 and IncHII, as well as the tehAtehB operon from the E. coli chromosome. When cells harbouring a Te(r) determinant were exposed to TeO3(2-), only a small fraction of the RSH content became oxidized. In addition to tellurite-dependent thiol oxidation, the resistance of E. coli mutants affected in proteins involved in disulfide-bond formation (dsb) was investigated. Mutant strains of dsbA and dsbB were found to be hypersensitive to tellurite (MIC 0.008-0.015 microg K2TeO3 ml(-1) compared to wild-type E. coli with MICs of 1-2 microg K2TeO3 ml(-1)). In contrast, dsbC and dsbD mutants showed no hypersensitivity. The results suggest that hypersensitivity to tellurite is reliant on the presence of an isomerase activity and not the thiol oxidase activity of the Dsb proteins. The results establish that the Te(r) determinants play an important role in maintaining homeostasis of the intracellular reducing environment within gram-negative cells through specific reactions with either TeO3(2-) or thiol:tellurium products.

Erwinia carotovora DsbA mutants: evidence for a periplasmic-stress signal transduction system affecting transcription of genes encoding secreted proteins

The dsbA genes, which encode major periplasmic disulfide-bond-forming proteins, were isolated from Erwinia carotovora subsp. carotovora (Ecc) and Erwinia carotovora subsp. atroseptica (Eca), and the dsbC gene, encoding another periplasmic disulfide oxidoreductase was isolated from Ecc. All three genes were sequenced and mutants deficient in these genes were created by marker exchange mutagenesis. The Ecc mutants were severely affected in activity and secretion of pectate lyase, probably due to the absence of functional PelC, which is predicted to require disulfide bond formation to achieve its correct conformation prior to secretion across the outer membrane. Similarly, endopolygalacturonase, also predicted to possess disulfide bonds, displayed reduced activity. The major Ecc cellulase (CelV) does not contain cysteine residues and was still secreted in dsbA-deficient strains. This observation demonstrated unequivocally that the localization and activity of the individual components of the Out apparatus are independent of disulfide bond formation. Surprisingly, cellulase activity was shown to be increased approximately two- to threefold in the DsbA mutant. This phenomenon resulted from transcriptional up-regulation of celV gene expression. In contrast, transcription of both pelC and peh were down-regulated in dsbA-deficient strains when compared to the wild-type. Protease (Prt) activity and secretion were unaffected in the Ecc dsbA mutant. Prt activity was considerably reduced in the double dsbA dsbC mutant. However Prt was secreted normally in this strain. The Eca dsbA mutant was found to be non-motile, suggesting that disulfide bond formation is essential for motility in this strain. All of the dsb mutants showed reduced tissue maceration in planta. These results suggest that a feedback regulation system operates in Ecc. In this system, defects in periplasmic disulfide bond formation act as a signal which is relayed to the transcription machinery regulating gene expression in diverse ways.

DsbA is required for stable expression of outer membrane protein YscC and for efficient Yop secretion in Yersinia pestis

The role of the periplasmic disulfide oxidoreductase DsbA in Yop secretion was investigated in Yersinia pestis. A Y. pestis dsbA mutant secreted reduced amounts of the V antigen and Yops and expressed reduced amounts of the full-sized YscC protein. Site-directed mutagenesis of the four cysteine residues present in the YscC protein resulted in defects similar to those found in the dsbA mutant. These results suggest that YscC contains at least one disulfide bond that is essential for the function of this protein in Yop secretion.

The protein disulphide isomerase gene of the fungus Trichoderma reesei is induced by endoplasmic reticulum stress and regulated by the carbon source

The gene pdi1 encoding protein disulphide isomerase was isolated from the filamentous fungus Trichoderma reesei by degenerate PCR based on a consensus PDI active-site sequence. It was shown that the Trichoderma pdi1 cDNA is able to complement a yeast mutant with a disrupted PDI1 gene. The putative T. reesei PD1I protein has a predicted 20-amino acid N-terminal signal sequence and the C-terminal fungal consensus ER retention signal HDEL. The mature protein shows strong conservation relative to other fungal protein disulphide isomerases. The T. reesei pdi1 promoter has two possible unfolded protein response (UPR) elements and it was shown by treatments with dithiothreitol and tunicamycin that the gene is under the control of the UPR pathway. Expression of a heterologous protein, an IgG antibody Fab fragment, in Trichoderma increases pdi1 expression, probably by inducing the UPR. The level of T. reesei pdi1 mRNA is also regulated by the carbon source, being lowest in glucose-containing media and highest on carbon sources that induce the genes encoding extracellular enzymes. The mechanism of this regulation was studied by examining pdi1 mRNA levels under conditions where the extracellular enzymes are induced by sophorose, as well as in the strain RutC-30, which is mutant for the glucose repressor gene cre1. The results suggest that neither sophorose induction nor glucose repression by the CREI protein affect the pdi1 promoter directly.

Oxidative protein folding is driven by the electron transport system

Disulfide bond formation is catalyzed in vivo by DsbA and DsbB. Here we reconstitute this oxidative folding system using purified components. We have found the sources of oxidative power for protein folding and show how disulfide bond formation is linked to cellular metabolism. We find that disulfide bond formation and the electron transport chain are directly coupled. DsbB uses quinones as electron acceptors, allowing various choices for electron transport to support disulfide bond formation. Electrons flow via cytochrome bo oxidase to oxygen under aerobic conditions or via cytochrome bd oxidase under partially anaerobic conditions. Under truly anaerobic conditions, menaquinone shuttles electrons to alternate final electron acceptors such as fumarate. This flexibility reflects the vital nature of the disulfide catalytic system.

Complementation of DsbA deficiency with secreted thioredoxin variants reveals the crucial role of an efficient dithiol oxidant for catalyzed protein folding in the bacterial periplasm

The thiol/disulfide oxidoreductase DsbA is the strongest oxidant of the thioredoxin superfamily and is required for efficient disulfide bond formation in the periplasm of Escherichia coli. To determine the importance of the redox potential of the final oxidant in periplasmic protein folding, we have investigated the ability of the most reducing thiol/disulfide oxidoreductase, E.coli thioredoxin, of complementing DsbA deficiency when secreted to the periplasm. In addition, we secreted thioredoxin variants with increased redox potentials as well as the catalytic a-domain of human protein disulfide isomerase (PDI) to the periplasm. While secreted wild-type thioredoxin and the most reducing thioredoxin variant could not replace DsbA, all more oxidizing thioredoxin variants as well as the PDI a-domain could complement DsbA deficiency in a DsbB-dependent manner. There is an excellent agreement between the activity of the secreted thioredoxin variants in vivo and their ability to oxidize polypeptides fast and quantitatively in vitro. We conclude that the redox potential of the direct oxidant of folding proteins and in particular its reactivity towards reduced polypeptides are crucial for efficient oxidative protein folding in the bacterial periplasm.

Mapping an interface of SecY (PrlA) and SecE (PrlG) by using synthetic phenotypes and in vivo cross-linking

SecY and SecE are integral cytoplasmic membrane proteins that form an essential part of the protein translocation machinery in Escherichia coli. Sites of direct contact between these two proteins have been suggested by the allele-specific synthetic phenotypes exhibited by pairwise combinations of prlA and prlG signal sequence suppressor mutations in these genes. We have introduced cysteine residues within the first periplasmic loop of SecY and the second periplasmic loop of SecE, at a specific pair of positions identified by this genetic interaction. The expression of the cysteine mutant pair results in a dominant lethal phenotype that requires the presence of DsbA, which catalyzes the formation of disulfide bonds. A reducible SecY-SecE complex is also observed, demonstrating that these amino acids must be sufficiently proximal to form a disulfide bond. The use of cysteine-scanning mutagenesis enabled a second contact site to be discovered. Together, these two points of contact allow the modeling of a limited region of quaternary structure, establishing the first characterized site of interaction between these two proteins. This study proves that actual points of protein-protein contact can be identified by using synthetic phenotypes.

Mutations in dsbA and dsbB, but not dsbC, lead to an enhanced sensitivity of Escherichia coli to Hg2+ and Cd2+

The Dsb proteins are involved in disulfide bond formation, reduction and isomerisation in a number of Gram-negative bacteria. Mutations in dsbA or dsbB, but not dsbC, increase the proportion of proteins with free thiols in the periplasm compared to wild-type. We investigated the effects of mutations in these genes on the bacterial resistance to mercuric and cadmium salts. Mutations in genes involved primarily in disulfide formation (dsbA and dsbB) generally enhanced the sensitivity to Hg2+ and Cd2+ while a mutation of the dsbC gene (primarily an isomerase of disulfide bonds) had no effect. Mutations of the dsb genes had no effect on the expression of the mercury-resistance determinants of the transposon Tn501.

A temperature-dependent switch from chaperone to protease in a widely conserved heat shock protein

Misfolding or unfolding of polypeptides can occur as a consequence of environmental stress and spontaneous mutation. The abundance of general chaperones and proteases suggests that cells distinguish between proteins that can be refolded and "hopeless" cases fated to enter the proteolytic pathway. The mechanisms controlling this key metabolic decision are not well understood. We show here that the widely conserved heat shock protein DegP (HtrA) has both general molecular chaperone and proteolytic activities. The chaperone function dominates at low temperatures, while the proteolytic activity is present at elevated temperatures. These results show that a single cellular factor can switch between two key pathways, controlling protein stability and turnover. Implications of this finding for intracellular protein metabolism are discussed.

A highly conserved nematode protein folding operon in Caenorhabditis elegans and Caenorhabditis briggsae

In the free-living model nematode, Caenorhabditis elegans, a protein-folding co-transcribed gene pair has previously been described. The degree and form of trans-splicing, orientation and spacing of the genes, and the co-ordinate co-expression of protein folding catalysts in the nematode's hypodermis indicated this to be a functionally important operon. This gene pair has now been cloned and compared in the related organism Caenorhabditis briggsae to identify evolutionarily conserved, functionally important features. The corresponding C. briggsae gene pair was found to share the operon-specific features, including sequence homology blocks in the upstream 5' flanking regions. The intergenic regions were not conserved. The homology block closest to the translational initiation codon of the upstream gene was found to contain a known Ceanorhabbitis promoter element site, and may therefore be an important cis-regulatory region directing the hypodermis-specific expression of this operon gene of C. elegans. This study also provides further confirmation of the high degree of chromosomal synteny between these nematode species.

Identification of Tn10 insertions in the dsbA gene affecting Escherichia coli biofilm formation

Escherichia coli was used as model to study initial adhesion and early biofilm development to an abiotic surface. Tn10 insertion mutants with reduced attachment to a polystyrene surface were isolated. Three adhesion mutants harbored the transposon in the dsbA gene, whose product, DsbA, catalyses folding of numerous extracytoplasmic disulfide bond-containing proteins. All three mutants were weakly adherent and grew poorly. Cell surface structure analysis showed that motility. type 1 fimbriation and lipopolysaccharide structure were affected in these mutants. The pleiotropic effect of the dsbA mutations on biofilm formation is discussed.

Random circular permutation of DsbA reveals segments that are essential for protein folding and stability

One of the key questions in protein folding is whether polypeptide chains require unique nucleation sites to fold to the native state. In order to identify possible essential polypeptide segments for folding, we have performed a complete circular permutation analysis of a protein in which the natural termini are in close proximity. As a model system, we used the disulfide oxidoreductase DsbA from Escherichia coli, a monomeric protein of 189 amino acid residues. To introduce new termini at all possible positions in its polypeptide chain, we generated a library of randomly circularly permuted dsbA genes and screened for active circularly permuted variants in vivo. A total of 51 different active variants were identified. The new termini were distributed over about 70 % of the polypeptide chain, with the majority of them occurring within regular secondary structures. New termini were not found in approximately 30 % of the DsbA sequence which essentially correspond to four alpha-helices of DsbA. Introduction of new termini into these "forbidden segments" by directed mutagenesis yielded proteins with altered overall folds and strongly reduced catalytic activities. In contrast, all active variants analysed so far show structural and catalytic properties comparable with those of DsbA wild-type. We suggest that random circular permutation allows identification of contiguous structural elements in a protein that are essential for folding and stability.

Functional properties of the two redox-active sites in yeast protein disulphide isomerase in vitro and in vivo

Protein folding catalysed by protein disulphide isomerase (PDI) has been studied both in vivo and in vitro using different assays. PDI contains a CGHC active site in each of its two catalytic domains (a and a'). The relative importance of each active site in PDI from Saccharomyces cerevisiae (yPDI) has been analysed by exchanging the active-site cysteine residues for serine residues. The activity of the mutant forms of yPDI was determined quantitatively by following the refolding of bovine pancreatic trypsin inhibitor in vitro. In this assay the activity of the wild-type yPDI is quite similar to that of human PDI, both in rearrangement and oxidation reactions. However, while the a domain active site of the human enzyme is more active than the a'-site, the reverse is the case for yPDI. This prompted us to set up an assay to investigate whether the situation would be different with a native yeast substrate, procarboxypeptidase Y. In this assay, however, the a' domain active site also appeared to be much more potent than the a-site. These results were unexpected, not only because of the difference with human PDI, but also because analysis of folding of procarboxypeptidase Y in vivo had shown the a-site to be most important. We furthermore show that the apparent difference between in vivo and in vitro activities is not due to catalytic contributions from the other PDI homologues found in yeast.

Baculovirus expression and biochemical characterization of the human microsomal triglyceride transfer protein

The microsomal triglyceride transfer protein (MTP) complexed to protein disulphide isomerase (PDI) is obligatory for the assembly of chylomicrons and very-low-density lipoproteins. The determination of the atomic structure of the MTP-PDI heterodimer has important implications for the treatment of those forms of hyperlipidaemia associated with the overproduction of very-low-density lipoproteins, which predispose to premature coronary heart disease. To perform structural studies of the human MTP-PDI complex it was necessary to produce milligram quantities of pure protein. We chose the baculovirus expression system for this purpose. Insects cells were co-infected with recombinant viruses encoding FLAG-tagged MTP and His-tagged PDI; the resulting heterodimer was purified by affinity chromatography. From 5 litres of insect cells, 4-6 mg of more than 95% pure recombinant protein was obtained. CD and attenuated total reflection Fourier-transform infrared spectroscopy indicate that the purified protein has around 34% alpha-helical and 33% beta-structure content. The recombinant protein had a comparable triglyceride transfer activity to that of bovine MTP-PDI. The production of polyclonal antibodies raised against the MTP and PDI subunits of the purified protein is described. The present study demonstrates the feasibility of expressing two proteins at high levels in insect cells and describes a transferable methodology for the purification of the resulting protein complex.

Autodegradation of protein disulfide isomerase

Protein disulfide isomerase (PDI) and its degradation products were found in HepG2, COS-1, and CHO-K1 cells. Whether or not the products were formed through autodegradation of PDI was examined, since PDI contains the CGHC motif, which is the active center of proteolytic activity in ER-60 protease. Commercial bovine PDI was autodegraded to produce a trimmed PDI. In addition, human recombinant PDI also had autodegradation activity. Mutant recombinant PDIs with CGHC motifs of which cysteine residues were replaced with serine or alanine residues were prepared. However, they were not autodegraded, suggesting the cysteine residues of motifs are necessary for autodegradation.

Cell-surface protein disulfide isomerase catalyzes transnitrosation and regulates intracellular transfer of nitric oxide

Since thiols can undergo nitrosation and the cell membrane is rich in thiol-containing proteins, we considered the possibility that membrane surface thiols may regulate cellular entry of NO. Recently, protein disulfide isomerase (PDI), a protein that catalyzes thio-disulfide exchange reactions, has been found on the cell-surface membrane. We hypothesized that cell-surface PDI reacts with NO, catalyzes S-nitrosation reactions, and facilitates NO transfer from the extracellular to intracellular compartment. We observed that PDI catalyzes the S-nitrosothiol-dependent oxidation of the heme group of myoglobin (15-fold increase in the rate of oxidation compared with control), and that NO reduces the activity of PDI by 73.1 +/- 21.8% (P < 0.005). To assess the role of PDI in the cellular action of NO, we inhibited human erythroleukemia (HEL) cell-surface PDI expression using an antisense phosphorothioate oligodeoxynucleotide directed against PDI mRNA. This oligodeoxynucleotide decreased cell-surface PDI content by 74.1 +/- 9.3% and PDI folding activity by 46.6 +/- 3.5% compared with untreated or "scrambled" phosphorothioate oligodeoxynucleotide-treated cells (P < 0.0001). This decrease in cell-surface PDI was associated with a significant decrease in cyclic guanosine monophosphate (cGMP) generation after S-nitrosothiol exposure (65.4 +/- 26.7% reduction compared with control; P < 0.05), with no effect on cyclic adenosine monophosphate (cAMP) generation after prostaglandin E1 exposure. These data demonstrate that the cellular entry of NO involves a transnitrosation mechanism catalyzed by cell-surface PDI. These observations suggest a unique mechanism by which extracellular NO gains access to the intracellular environment.

In vivo formation of Cu,Zn superoxide dismutase disulfide bond in Escherichia coli

We have found that the in vivo folding of periplasmic Escherichia coli Cu,Zn superoxide dismutase is assisted by DsbA, which catalyzes the efficient formation of its single disulfide bond, whose integrity is essential to ensure full catalytic activity to the enzyme. In line with these findings, we also report that the production of recombinant Xenopus laevis Cu,Zn superoxide dismutase is enhanced when the enzyme is exported in the periplasmic space or is expressed in thioredoxin reductase mutant strains. Our data show that inefficient disulfide bond oxidation in the bacterial cytoplasm inhibits Cu,Zn superoxide dismutase folding in this cellular compartment.

Redox control of exofacial protein thiols/disulfides by protein disulfide isomerase

Protein disulfide isomerase (PDI) facilitates proper folding and disulfide bonding of nascent proteins in the endoplasmic reticulum and is secreted by cells and associates with the cell surface. We examined the consequence of over- or underexpression of PDI in HT1080 fibrosarcoma cells for the redox state of cell-surface protein thiols/disulfides. Overexpression of PDI resulted in 3.6-4. 2-fold enhanced secretion of PDI and 1.5-1.7-fold increase in surface-bound PDI. Antisense-mediated underexpression of PDI caused 38-53% decreased secretion and 10-33% decrease in surface-bound PDI. Using 5,5'-dithio-bis(2-nitrobenzoic acid) to measure surface protein thiols, a 41-50% increase in surface thiols was observed in PDI-overexpressing cells, whereas a 29-33% decrease was observed in underexpressing cells. Surface thiol content was strongly correlated with cellular (r = 0.998) and secreted (r = 0.969) PDI levels. The pattern of exofacial protein thiols was examined by labeling with the membrane-impermeable thiol reactive compound, 3-(N-maleimidylpropionyl)biocytin. Fourteen identifiable proteins on HT1080 cells were labeled with 3-(N-maleimidylpropionyl)biocytin. The intensity of labeling of 11 proteins was increased with overexpression of PDI, whereas the intensity of labeling of 3 of the 11 proteins was clearly decreased with underexpression of PDI. These findings indicated that secreted PDI was controlling the redox state of existing exofacial protein thiols or reactive disulfide bonds.

The acidic C-terminal domain of protein disulfide isomerase is not critical for the enzyme subunit function or for the chaperone or disulfide isomerase activities of the polypeptide

Protein disulfide isomerase (PDI) is a multifunctional polypeptide that acts as a subunit in the animal prolyl 4-hydroxylases and the microsomal triglyceride transfer protein, and as a chaperone that binds various peptides and assists their folding. We report here that deletion of PDI sequences corresponding to the entire C-terminal domain c, previously thought to be critical for chaperone activity, had no inhibitory effect on the assembly of recombinant prolyl 4-hydroxylase in insect cells or on the in vitro chaperone activity or disulfide isomerase activity of purified PDI. However, partially overlapping critical regions for all these functions were identified at the C-terminal end of the preceding thioredoxin-like domain a'. Point mutations introduced into this region identified several residues as critical for prolyl 4-hydroxylase assembly. Circular dichroism spectra of three mutants suggested that two of these mutations may have caused only local alterations, whereas one of them may have led to more extensive structural changes. The critical region identified here corresponds to the C-terminal alpha helix of domain a', but this is not the only critical region for any of these functions.

On the role of the cis-proline residue in the active site of DsbA

In addition to the Cys-Xaa-Xaa-Cys motif at position 30-33, DsbA, the essential catalyst for disulfide bond formation in the bacterial periplasm shares with other oxidoreductases of the thioredoxin family a cis-proline in proximity of the active site residues. In the variant DsbA(P151A), this residue has been changed to an alanine, an almost isosteric residue which is not disposed to adopt the cis conformation. The substitution strongly destabilized the structure of DsbA, as determined by the decrease in the free energy of folding. The pKa of the thiol of Cys30 was only marginally decreased. Although in vivo the variant appeared to be correctly oxidized, it exhibited an activity less than half that of the wild-type enzyme with respect to the folding of alkaline phosphatase, used as a reporter of the disulfide bond formation in the periplasm. DsbA(P151A) crystallized in a different crystal form from the wild-type protein, in space group P2(1) with six molecules in the asymmetric unit. Its X-ray structure was determined to 2.8 A resolution. The most significant conformational changes occurred at the active site. The loop 149-152 adopted a new backbone conformation with Ala151 in a trans conformation. This rearrangement resulted in the loss of van der Waals interactions between this loop and the disulfide bond. His32 from the Cys-Xaa-Xaa-Cys sequence presented in four out of six molecules in the asymmetric unit a gauche conformation not observed in the wild-type protein. The X-ray structure and folding studies on DsbA(P151A) were consistent with the cis-proline playing a major role in the stabilization of the protein. A role for the positioning of the substrate is discussed. These important properties for the enzyme function might explain the conservation of this residue in DsbA and related proteins possessing the thioredoxin fold.

Conversion of a catalytic into a structural disulfide bond by circular permutation

The thiol-disulfide oxidoreductase DsbA from Escherichia coli is the strongest oxidant of the enzyme family and required for disulfide bond formation in the bacterial periplasm. The catalytic domain of this 189-residue protein has a thioredoxin-like fold and contains a catalytic disulfide bridge that is located within the sequence Cys30-Pro31-His32-Cys33 at the N-terminus of an alpha-helix. The Cys30-Cys33 disulfide bond destabilizes DsbA by about 16 kJ/mol at pH 7.0, which appears to be caused by the extremely low pKa value of approximately 3.4 of the nucleophilic Cys30 thiol. Here we report the characterization of a circularly permuted variant of DsbA, termed H32-P31, in which the natural termini are connected by a Gly3-Thr-Gly linker and the new termini are located between the active-site cysteines (first residue His32, last residue Pro31). The disulfide bond in the variant thus connects the second with the penultimate residue. H32-P31 adopts a wild-type-like structure and folds reversibly and cooperatively in both redox forms. However, the permuted variant is catalytically inactive as dithiol oxidase in vivo and in vitro. Both cysteine thiols have pKa values > 8; the variant is 500-fold more reducing than the wild type and more stable in its oxidized form. Thus, the Cys30-Cys33 disulfide in the variant H32-P31 has adopted properties of a structural disulfide bond.

Improved folding yields of a model protein using protein disulfide isomerase

PURPOSE

To study the effects of recombinant human protein disulfide isomerase (rhPDI) concentration, reduced glutathione:oxidized glutathione ratio (GSH:GSSG) and temperature on the efficiency of oxidative folding of a model protein, recombinant human interleukin 2 (C125A mutation) (C125A rhIL-2).

METHODS

C 125A rhIL-2 inclusion bodies were reduced and denatured by guanidium hydrochloride (Gdm.Cl) and 100 mM GSH. The solution was diluted 10 times into folding buffer, allowing C125A rhIL-2 to fold either in the absence or presence of rhPDI. The renatured and unfolded C125A rhIL-2 species were quantitated by reversed phase-HPLC.

RESULTS

The initial folding rate of C125A rhIL-2 linearly increased with rhPDI:C125A rhIL-2 molar ratio in the first 2.5 minutes, and reached the highest rate when the rhPDI:C125A rhIL-2 ratio was 1:1. The oxidative folding of C125A rhIL-2 linearly increased as the GSH:GSSG molar ratio decreased from 10:0 to 10:3. The folding of C125A rhIL-2 was also dependent on temperature, and optimum folding was realized at 23 degrees C.

CONCLUSIONS

These results demonstrate that under optimal redox potential and temperature, rhPDI enhances the oxidative folding of C125A rhIL-2. In the oxidative folding of C125A rhIL-2, rhPDI exerts its effect on folding by the acceleration of thiol/disulfide interchange.

Thioreductase activity of retina cognin and its role in cell adhesion

Retina cognin (R-cognin) is a 50-kDa protein on the surface of embryonic chick retina cells that mediates cell-cell recognition and neuronal differentiation. It is developmental stage- and tissue-specific in its expression. The partial cDNA clone for R-cognin is nearly identical to that of chicken protein disulfide isomerase (chicken PDI) and enzyme with thioreductase activity. The R-cognin clone extends from beyond the 3' polyadenylation site up to the boundary between PDI exons 1 and 2, with the putative R-cognin equivalent of PDI exon 1 remaining uncloned. The question posed here was whether the sequence-specific properties of PDI were significant in the action of R-cognin. We show that R-cognin, like PDI, has thioreductase activity as revealed by RNase renaturation enzymatic assays. We then asked if this thioreductase activity was involved in the mediation of cell adhesion and recognition in developing chick retina. We show, through cell aggregation assays, that both R-cognin and chicken PDI enhance chick retina cell aggregation but not that of cells from other CNS tissues. We also show that treating R-cognin and chicken PDI with the thioreductase inhibitor 5,5'-dithio-bis (2-nitrobenzoic acid), which covalently binds to the functional cysteines of the thioreductase active sites, reduces the enhancement of cell aggregation. Thus R-cognin acts, in part, by catalyzing a covalent protein-protein linkage at the cell surface.

Fusion expression of human pro-urokinase with E. coli thioredoxin

Human pro-urokinase (pro-UK) was cloned into plasmid pET32b and fused to the E. coli thioredoxin (trxA). When expressed in E. coli AD494(DE3), the fusion protein Trx-pro-UK accumulated as insoluble inclusion bodies and amounted to 35% of total cellular proteins. When co-expressed with molecular chaperones human protein disulfide isomerase (PDI) and E. coli GroESL, all the expressed products still existed in the form of insoluble inclusion bodies.

Increased solubility of glutathione S-transferase-P16 (GST-p16) fusion protein by co-expression of chaperones groes and groel in Escherichia coli

Human cdk (cyclin dependent kinase) inhibitor p16 was fused with glutathione S-transferase (GST) and the GST-p16 fusion protein is under the control of T7 promoter. When expressed in E. coli BL21(DE3), most products existed in the form of insoluble inclusion bodies. When co-expressed with molecular chaperones E. coli GroESL, most GST-p16 products accumulated in the soluble form with a 5-6 fold increase in solubility. When coproduced with human protein disulfide isomerase (PDI), there was no improvement in the solubility of GST-p16 fusion protein.

A recombinant protein disulfide isomerase homologue from Ancylostoma caninum

The objective of this study was to characterize a recombinant antigen of Ancylostoma caninum that had been identified by immunoscreening with selected antisera as described elsewhere. In vitro expression of clone Ac38-1 produced a protein with an apparent molecular mass of approximately 38 kDa, which reacted in Western blots with the antiserum from rabbits experimentally infected with L3 and also with affinity-purified antibodies against hydrophilic proteins of the cephalic glands obtained from the antiserum against the intestine, cephalic glands, and cervical glands of adult worms. It was recognized not by antisera from dogs percutaneously infected with 1,000 L3 of A. caninum but by antiserum from dogs infected with 100,000 L3 of A. caninum. DNA sequencing of clone Ac38-1 showed a cDNA fragment with a coding region of 1,014 bp. Comparison of clone Ac38-1 with the Genbank DNA data base revealed 78% identity with a 244-bp segment of the cm5b5 clone of the free-living nematode Caenorhabditis elegans coding for a protein disulfide isomerase gene. The deduced amino acid sequence of clone Ac38-1 showed 82% identity with a 334-amino-acid (aa) segment of the protein disulfide isomerase of C. elegans and 73% identity with a 334-aa segment of the protein disulfide isomerase aa sequence of Onchocerca volvulus.

Molecular evolution of the domain structures of protein disulfide isomerases

Protein disulfide isomerase (PDI) is an enzyme that promotes protein folding by catalyzing disulfide bridge isomerization. PDI and its relatives form a diverse protein family whose members are characterized by thioredoxin-like (TX) domains in the primary structures. The family was classified into four classes by the number and the relative positions of the TX domains. To investigate the evolution of the domain structures, we aligned the amino acid sequences of the TX domains, and the molecular phylogeny was examined by the NJ and ML methods. We found that all of the current members of the PDI family have evolved from an ancestral enzyme, which has two TX domains in the primary structure. The diverse domain structures of the members have been generated through domain duplications and deletions.

Inactivation of DsbA, but not DsbC and DsbD, affects the intracellular survival and virulence of Shigella flexneri

In this study, three mutants, dsbA::kan, dsbC-kan, and dsbD-kan, of Shigella flexneri serotype 5 were constructed and characterized to investigate the role of the periplasmic thiol:disulfide oxidoreductases in pathogenicity. In gentamicin protection assays and the Serény test, the dsbA mutant showed reduced virulence while the dsbC and dsbD mutants were similar to the wild type. That inactivation of dsbA was responsible for the reduced virulence was verified by complementation with the cloned wild-type gene in in vitro and in vivo assays. Despite the changed virulence behavior, the dsbA mutant could penetrate HeLa cells 15 min postinfection, consistent with the fact that it actively secretes Ipa proteins upon Congo red induction. Furthermore, the dsbA mutant was able to produce actin comets and protrusions, indicating its capacity for intra- and intercellular spread. However, a kinetic analysis of intracellular growth showed that the dsbA mutant barely grew in HeLa cells during a 4-h infection whereas the wild type had a doubling time of 41 min. Electron microscopy analysis revealed that dsbA mutant bacteria were trapped in protrusion-derived vacuoles surrounded by double membranes, resembling an icsB mutant reported previously. Moreover, the trapped bacteria appeared to be lysed simultaneously with the double membranes, resulting in characteristic empty vacuoles in the host cell cytosol. Thus, the attenuation mechanism for dsbA mutant appears to be more complicated than was previously suggested.

ERp28, a human endoplasmic-reticulum-lumenal protein, is a member of the protein disulfide isomerase family but lacks a CXXC thioredoxin-box motif

We report on the isolation, sequence and a putative role of a human endoplasmic-reticulum-lumenal protein, ERp28. The protein has the C-terminal retention signal KEEL and localizes to the endoplasmic reticulum (ER) as seen by subcellular fractionation and immunofluorescence studies. The protein has significant sequence similarity to members of the protein disulfide isomerase (PDI) family, although it lacks the thioredoxin box (CGHC) motif. We propose, on the basis of sequence analysis, a model of the domain structure of PDI, representing a significant extension of previously proposed models. Our results are in partial agreement with recently published NMR data [Kemmink, J., Darby, J., Dijkstra, K., Nilges, M. & Creighton, T. E. (1997) Curr. Biol. 7, 239-245] and indicate that PDI contains, in addition to the two thioredoxin folds described in previous models, two thioredoxin folds within the domains previously defined as b and b'. The thioredoxin domain of ERp28 shares a higher degree of similarity with the corresponding active and inactive domains of PDI than with other members of the PDI family, indicating that ERp28 developed from an ancient form of PDI or a PDI precursor. In contrast to Ig-heavy-chain-binding protein, human ERp28 is not induced by metabolic stress (tunicamycin). In in vitro experiments, ERp28 and calnexin precipitate with overexpressed, wild-type hepatitis B small surface antigen and with a mutated ER-retained form. This indicates that ERp28, as calnexin, may be involved in the processing of secretory proteins within the ER.

Increasing the secretory capacity of Saccharomyces cerevisiae for production of single-chain antibody fragments

We have produced single-chain antibody fragments (scFv) in Saccharomyces cerevisiae at levels up to 20 mg/L in shake flask culture by a combination of expression level tuning and overexpression of folding assistants. Overexpression of the chaperone BiP or protein disulfide isomerase (PDI) increases secretion titers 2-8 fold for five scFvs. The increases occur for scFv expression levels ranging from low copy to ER-saturating overexpression. The disulfide isomerase activity of PDI, rather than its chaperone activity, is responsible for the secretion increases. A synergistic increase in scFv production occurs upon cooverexpression of BiP and PDI.

Protein folding stability can determine the efficiency of escape from endoplasmic reticulum quality control

A fraction of each secreted protein is retained and degraded by the endoplasmic reticulum (ER) quality control apparatus that restricts export to correctly folded proteins. The intrinsic biophysical attributes that determine efficiency of escape from this proofreading process have been examined by expressing mutants of bovine pancreatic trypsin inhibitor (BPTI) in yeast. Secretion efficiency is strongly correlated with thermodynamic stability for a series of six point mutations of BPTI. No correlation of secretion efficiency with either oxidative folding or refolding rates in vitro is found; both the rapidly folded Y35L BPTI mutant and the slowly unfolded G36D BPTI mutant exhibit low secretion efficiency. Elimination of cysteines 14 and 38 by mutagenesis does not increase secretion efficiency, indicating that intramolecular thiol/disulfide rearrangements are not primarily responsible for retention and degradation of destabilized BPTI variants. Mutant yeast strains with diminished ER-associated degradation do not secrete BPTI more efficiently, indicating that retention and degradation are separable processes. These data support a model for ER quality control, wherein protein folding is functionally reversible and the relative rates of folding, unfolding, vesicular export, and retention determine secretion efficiency.

Improved folding of apo-retinol-binding protein in the periplasm of Escherichia coli: positive influences of dsbC coexpression and of an amino acid exchange in the vitamin A binding site

The in vivo folding of the serum retinol-binding protein (RBP), a representative of the lipocalin structural family, is known to be complex. In order to gain insight into the essential steps along its folding pathway the heterologous production of the functional protein in Escherichia coli was investigated. Simultaneous overexpression of the bacterial dsbC gene, which codes for a periplasmic thiol-disulphide oxidoreductase, prevented the formation of soluble RBP variants with non-native disulphide bonds that were otherwise observed. Although the coexpression of dsbC had furthermore a stabilizing effect on the cell viability, the relative yield of the solubly produced RBP was not much better. In an attempt to enhance its folding efficiency, a favourable point mutation in the inner part of the retinol-binding pocket was predicted. Replacement of the polar Gln117 with an lie side chain seemed not only to relieve the unfavourable energetics of the carboxamide group in the environment of predominantly non-polar residues but also to fill an adjacent cavity in the hydrophobic core. Indeed, this single substitution reproducibly resulted in a more than threefold increase in the amount of functional recombinant RBP. Ligand binding experiments showed that the affinity of this mutant for retinol was slightly enhanced. Kinetic measurements revealed that this was due to a higher association rate whereas the dissociation of the complex with retinol was essentially unaffected. Although the question remained why nature did not select this obviously beneficial mutation, our results demonstrate that the folding pathway of a lipocalin can be optimized by protein engineering.

A single dipeptide sequence modulates the redox properties of a whole enzyme family

BACKGROUND

Disulfide exchange reactions are catalyzed by thiol/disulfide oxidoreductases. These enzymes possess a thioredoxin fold and contain a catalytic disulfide with the sequence Cys-X-X-Cys at the N terminus of an alpha helix. Despite these similarities, the various members differ strongly in their redox potentials (-122 mV to -270 mV). Using the strong oxidant DsbA from Escherichia coli as a model system, we investigated whether the redox properties of these enzymes can be modulated rationally by exchange of the X-X dipeptide.

RESULTS

The X-X dipeptide of DsbA (Cys30-Pro31-His32-Cys33) was exchanged by the dipeptides of eukaryotic protein disulfide isomerase (PDI; Gly-His), glutaredoxin (Pro-Tyr), and thioredoxin (Gly-Pro) from E. coli. All variants were less oxidizing than wild-type DsbA and their redox potentials were in the order of the related natural enzymes (DsbA > PDI > glutaredoxin > thioredoxin). The equilibrium constant between glutathione and the thioredoxin-like variant increased 1200-fold compared with wild-type DsbA. The variants also showed a strong increase in the pKa of the nucleophilic cysteine (Cys30). As for glutaredoxin and thioredoxin, the catalytic disulfide stabilized the corresponding variants while destabilizing wild-type DsbA and the PDI-like variant.

CONCLUSIONS

The X-X dipeptide in the active site of thiol/disulfide oxidoreductases appears to be the main determinant of the redox properties of these enzymes. This empirical finding should be very useful for the design of new thiol/disulfide oxidoreductases with altered redox potentials and for studying the function of these enzymes in vivo.

Enzymatic activation of immunoglobulin binding factor in female reproductive tract

Human seminal plasma and cervical mucus contains an immunoglobulin binding factor (IgBF) which interacts with IgG as monomers under reducing condition. It may play a role in preventing antibody production against allogeneic sperms in the female reproductive tract. However, since IgBF is secreted as a homodimer that does not bind IgG, in vivo activation systems should be investigated. GSH reduces the inactive native dimer to the active monomer. Protein disulfide isomerase (PDI), a molecular chaperone, alters the configuration of dimers to active monomers. 20S proteasomes produced by activated T cells which cleave the dimers in the presence of GSH to active fragments. All these activating systems are widely distributed as cellular enzymes in vivo. Also PDI mRNAs are expressed in uterine cervix, endometrium and fallopian tube. Since these enzymes are produced upon stimulation by the immune system, we hypothesize that immunocompetent cells interact with allogeneic sperms, leading to the local production of these enzymes that will activate IgBF.

Collagen hydroxylases and the protein disulfide isomerase subunit of prolyl 4-hydroxylases

Prolyl 4-hydroxylases catalyze the formation of 4-hydroxyproline in collagens and other proteins with an appropriate collagen-like stretch of amino acid residues. The enzyme requires Fe(II), 2-oxoglutarate, molecular oxygen, and ascorbate. This review concentrates on recent progress toward understanding the detailed mechanism of 4-hydroxylase action, including: (a) occurrence and function of the enzyme in animals; (b) general molecular properties; (c) intracellular sites of hydroxylation; (d) peptide substrates and mechanistic roles of the cosubstrates; (e) insights into the development of antifibrotic drugs; (f) studies of the enzyme's subunits and their catalytic function; and (g) mutations that lead to Ehlers-Danlos Syndrome. An account of the regulation of collagen hydroxylase activities is also provided.

Overexpression of Escherichia coli oxidoreductases increases recombinant insulin-like growth factor-I accumulation

Transient overexpression of either DsbA or DsbC can double the yield of periplasmic insulin-like growth factor (IGF)-I in Escherichia coli to 8.5 g/liter. Strikingly, most of the overexpressed DsbA or DsbC is found in the reduced form, implying that enhanced disulfide isomerization is responsible for the substantial increase in IGF-I yield. All of the accumulated IGF-I has had the signal sequence removed, underscoring the secretion capacity of this organism as well as its utility for efficient production of polypeptide with the correct amino terminus. The overexpressed IGF-I constitutes approximately 30% of the total cell protein. Overproduction of active site mutants of DsbA instead of the wild-type gene do not produce this increase in yield. With wild-type levels of DsbA and DsbC, most of the secreted IGF-I is found in disulfide-linked aggregates, although 10% is soluble and about 5% is correctly folded. Contrary to expectations, overexpression of the disulfide oxidoreductases decreased the soluble fraction. Because the aggregated protein can be efficiently solubilized and refolded, the increased yield is a significant benefit for the production of IGF-I.

The b' domain provides the principal peptide-binding site of protein disulfide isomerase but all domains contribute to binding of misfolded proteins

Protein disulfide isomerase (PDI) is a very efficient catalyst of folding of many disulfide-bonded proteins. A great deal is known about the catalytic functions of PDI, while little is known about its substrate binding. We recently demonstrated by cross-linking that PDI binds peptides and misfolded proteins, with high affinity but broad specificity. To characterize the substrate-binding site of PDI, we investigated the interactions of various recombinant fragments of human PDI, expressed in Escherichia coli, with different radiolabelled model peptides. We observed that the b' domain of human PDI is essential and sufficient for the binding of small peptides. In the case of larger peptides, specifically a 28 amino acid fragment derived from bovine pancreatic trypsin inhibitor, or misfolded proteins, the b' domain is essential but not sufficient for efficient binding, indicating that contributions from additional domains are required. Hence we propose that the different domains of PDI all contribute to the binding site, with the b' domain forming the essential core.

Secretion efficiency in Saccharomyces cerevisiae of bovine pancreatic trypsin inhibitor mutants lacking disulfide bonds is correlated with thermodynamic stability

Bovine pancreatic trypsin inhibitor (BPTI) has been widely used as a model protein to investigate protein structure and folding pathways. To study the role of its three disulfide bonds in folding, proofreading, and secretion of BPTI in an intact eucaryotic cell, BPTI was expressed and secreted from a synthetic gene in the yeast Saccharomyces cerevisiae. Site-directed mutagenesis was used to create all possible single and pairwise cysteine to alanine BPTI mutants, and the effect of these mutations on secretion efficiency was determined. The 5-55 disulfide bond is found to be essential for secretion-loss of either Cys5, Cys55, or both prevents secretion. Removal of the 14-38 disulfide bond results in a small reduction of secretion, but individual Cys14 or Cys38 replacements reduce secretion efficiency by 30%. Cys30 and Cys30-51 mutants are secreted at half the level of wild-type BPTI, while secretion of the Cys51 mutant is reduced by 90%. BPTI containing only a single disulfide bond (5-55) is not secreted. No relationship is observed between secretion efficiency and in vitro folding or unfolding rates, but mutant BPTI secretion is directly correlated with the in vitro unfolding temperature Tm and the free energy of stabilization provided by each of the three disulfides. These results indicate that structural fluctuations rather than the time-averaged structure observed by NMR or X-ray crystallography may determine recognition of a protein as misfolded and subsequent retention and degradation.

Modeling charge interactions and redox properties in DsbA

Accurate prediction of charge interactions in macromolecules presents a significant challenge for computational biology. A model for the low Cys30 pKa and oxidizing power of DsbA (Gane, P. J., Freedman, R. B., and Warwicker, J. (1995) J. Mol. Biol. 249, 376-387) has been investigated experimentally (Hennecke, J., Spleiss, C., and Glockshuber, R. (1997) J. Biol. Chem. 272, 189-195), with substitutions for Glu37 and Glu38 and with residues 38-40 removed. Measured changes in Cys30 pKa and redox potential were relatively small and reported to be in contrast to model predictions. It is now shown, particularly with calculations of wild-type:mutant differences for a range of salt concentrations, that the data are consistent with the model and support the key finding that a number of different factors contribute to the oxidizing power of DsbA, so that any particular one need not necessarily be large. A feature of the model is a low protein dielectric, and higher values (which are becoming popular in predictions of pH dependence) are inconsistent with both the difference data and the wild-type Cys30 pKa.

An ERp60-like protein from the filarial parasite Dirofilaria immitis has both transglutaminase and protein disulfide isomerase activity

Transglutaminases (TGases; EC 2.3.2.13) are a family of enzymes that catalyze calcium-dependent covalent cross-linking of cellular proteins by establishing epsilon-(gamma-glutamyl)lysine isopeptide bonds. These covalent isopeptide bonds are of great physiological significance because they are highly resistant to proteolysis, denaturants, and reducing agents. Prior studies have demonstrated the presence of isopeptide bonds in the sheath and cuticle of filarial parasites, suggesting an important role for TGase-catalyzed reactions during the growth and development of filarial nematodes. Herein we report the identification and cloning of a cDNA encoding a TGase from the dog heartworm Dirofilaria immitis (DiTG). The DiTG expressed in Escherichia coli (recombinant DiTG) was able to catalyze calcium-dependent cross-linking reactions. The derived amino acid sequence of the DiTG cDNA (pDiTG) predicts a protein of 57.1 kDa and includes an N-terminal hydrophobic signal peptide. The pDiTG has no sequence similarity with any of the known TGases, but it has significant homology to protein disulfide isomerase (PDI) and, particularly, to the PDI-related endoplasmic reticulum protein ERp60, a PDI isoform found in the lumen of endoplasmic reticulum. As predicted from the amino acid sequence homology, recombinant DiTG catalyzed the isomerization of intramolecular disulfide/sulfhydryl bonds in denatured RNase in vitro as effectively as did mammalian PDI. Conversely, purified PDI from bovine liver could catalyze protein cross-linking reactions in a Ca(2+)-dependent manner. This report describes the dual catalytic activity of TGase and PDI in post- and/or cotranslational modification of newly synthesized proteins. These TGase-catalyzed posttranslational modifications may play a pivotal role in the synthesis of new cuticle during the growth and maturation of filarial parasites.

Contributions of substrate binding to the catalytic activity of DsbC

DsbA and DsbC are involved in protein disulfide bond formation in the periplasm of Gram-negative bacteria. The two proteins are thought to fulfill different functions in vivo, DsbA as a catalyst of disulfide bond formation and DsbC as a catalyst of disulfide bond rearrangement. To explore the basis of this catalytic complementarity, the reaction mechanism of DsbC has been examined using unstructured model peptides that contain only one or two cysteine residues as substrates. The reactions between the various forms of the peptide and DsbC occur at rates up to 10(6)-fold faster than those that involve glutathione and DsbC, and they were constrained to occur at only one sulfur atom of disulfide bonds involving the peptide. Mixed disulfide complexes of DsbC and the peptide were 10(4)-fold more stable than the corresponding mixed disulfides with glutathione. These observations suggest that noncovalent binding interactions occur between the peptide and DsbC, which contribute to the very rapid kinetics of substrate utilization. The interactions between DsbC and the peptide appear to be more substantial than those between DsbA and the same peptide. The differences in the reaction of the peptide at the active sites of DsbA and DsbC provide insight into why DsbC is the better catalyst of disulfide bond rearrangement and how the active site chemistry of these structurally related proteins has been adapted to fulfill complementary functions.

Dependence of the anti-chaperone activity of protein disulphide isomerase on its chaperone activity

Protein disulphide isomerase (PDI) shows chaperone and anti-chaperone activities in assisting refolding of denatured and reduced lysozyme in redox Hepes buffer, but only chaperone activity in phosphate buffer and redox Hepes buffer containing 0.1 M NaCl. In non-redox Hepes buffer its anti-chaperone activity is very weak. PDI displays its anti-chaperone activity only for those substrates showing relatively low aggregation during refolding, and is strongly dependent on refolding conditions, of which ionic strength appears to be an important factor. The S-methylated PDI, fully active as a chaperone but devoid of isomerase activity, by itself shows only anti-chaperone activity, but reinforces rather than suppresses the chaperone activity of native PDI in the refolding of lysozyme. A fragment of PDI with the C-terminal peptide-binding sequence removed and devoid of chaperone activity does not show anti-chaperone activity in lysozyme refolding. It appears that the anti-chaperone activity of PDI is dependent on its chaperone activity.

Protein disulfide isomerase as a regulator of chloroplast translational activation

Light-regulated translation of chloroplast messenger RNAs (mRNAs) requires trans-acting factors that interact with the 5' untranslated region (UTR) of these mRNAs. Chloroplast polyadenylate-binding protein (cPABP) specifically binds to the 5'-UTR of the psbA mRNA and is essential for translation of this mRNA. A protein disulfide isomerase that is localized to the chloroplast and copurifies with cPABP was shown to modulate the binding of cPABP to the 5'-UTR of the psbA mRNA by reversibly changing the redox status of cPABP through redox potential or adenosine 5'-diphosphate-dependent phosphorylation. This mechanism allows for a simple reversible switch regulating gene expression in the chloroplast.

Dictyostelium discoideum protein disulfide isomerase, an endoplasmic reticulum resident enzyme lacking a KDEL-type retrieval signal

The primary activity of protein disulfide isomerase (PDI), a multifunctional resident of the endoplasmic reticulum (ER), is the isomerization of disulfide bridges during protein folding. We isolated a cDNA encoding Dictyostelium discoideum PDI (Dd-PDI). Phylogenetic analyses and basic biochemical properties indicate that it belongs to a subfamily called P5, many members of which differ from the classical PDIs in many respects. They lack an intervening inactive thioredoxin module, a C-terminal acidic domain involved in Ca2+ binding and a KDEL-type retrieval signal. Despite the absence of this motif, the ER is the steady-state location of Dd-PDI, suggesting the existence of an alternative retention mechanism for P5-related enzymes.

Assembly of human prolyl 4-hydroxylase and type III collagen in the yeast pichia pastoris: formation of a stable enzyme tetramer requires coexpression with collagen and assembly of a stable collagen requires coexpression with prolyl 4-hydroxylase

Prolyl 4-hydroxylase, the key enzyme of collagen synthesis, is an alpha2beta2 tetramer, the beta subunit of which is protein disulfide isomerase (PDI). Coexpression of the human alpha subunit and PDI in Pichia produced trace amounts of an active tetramer. A much higher, although still low, assembly level was obtained using a Saccharomyces pre-pro sequence in PDI. Coexpression with human type III procollagen unexpectedly increased the assembly level 10-fold, with no increase in the total amounts of the subunits. The recombinant enzyme was active not only in Pichia extracts but also inside the yeast cell, indicating that Pichia must have a system for transporting all the cosubstrates needed by the enzyme into the lumen of the endoplasmic reticulum. The 4-hydroxyproline-containing procollagen polypeptide chains were of full length and formed molecules with stable triple helices even though Pichia probably has no Hsp47-like protein. The data indicate that collagen synthesis in Pichia, and probably also in other cells, involves a highly unusual control mechanism, in that production of a stable prolyl 4-hydroxylase requires collagen expression while assembly of a stable collagen requires enzyme expression. This Pichia system seems ideal for the high-level production of various recombinant collagens for numerous scientific and medical purposes.

Cyclophilin and protein disulfide isomerase genes are co-transcribed in a functionally related manner in Caenorhabditis elegans

The ubiquitous enzymes peptidyl prolyl cis-trans isomerase (PPI, EC 5.2.1.8) and protein disulfide isomerase (PDI, EC 5.3.4.1) are important rate-limiting catalysts of protein-folding events in the cell. In the free-living nematode Caenorhabditis elegans, two genes encoding these enzymes (cyp-9 and pdi-1, respectively) are clustered together on chromosome III. In work described elsewhere, the encoded enzymes have been expressed as recombinant proteins and have been determined to possess in vitro PPI and PDI activity. Taken together, this organization of the two genes and the related functions of their transcripts indicate that they may be cotranscribed as a polycistronic unit, similar to bacterial operons. This study details the very close linkage of pdi-1 and cyp-9, which are in the same orientation. pdi-1 is the upstream gene, and the putative polyadenylation cleavage signal of this gene is separated from the trans-splice acceptor site of cyp-9 by only 103 bp. pdi-1 is trans-spliced by the ubiquitous nematode trans-spliced leader SL1, whereas cyp-9 was found to be predominantly trans-spliced by the "operon-specific" trans-spliced leader SL2. Similar trends in relative transcript abundance were demonstrated with synchronously produced mRNA for both genes during larval development, supporting the contention that the genes are co-expressed. Finally, reporter gene analysis provides strong evidence that both genes are controlled by a single upstream regulatory element, which directs expression of both enzymes in the hypodermal cells that synthesize the cuticle.

The enzymatic and non-enzymatic roles of protein-disulfide isomerase in apolipoprotein B secretion

Secretion of apolipoprotein B (apoB) from mammalian cells requires the presence of functional microsomal triglyceride transfer protein (MTP). We previously reported that co-expressing the human intestinal form of apoB, B48, with both subunits of human MTP in oleate-treated Sf21 cells led to a dramatic induction of B48 secretion. Deletion mutagenesis studies showed that the cysteine-enriched amino terminus of apoB was necessary for the MTP responsiveness (Gretch, D. G., Sturley, S. L., Wang, L., Dunning, A., Grunwald, K. A. A., Wetterau, J. R., Yao, Z., Talmud, P., and Attie, A. D. (1996) J. Biol. Chem. 271, 8682-8691). We therefore hypothesized that the small subunit of MTP, protein-disulfide isomerase (PDI), plays a role in apoB secretion by facilitating correct disulfide bond formation. To determine whether the enzymatic activities of PDI are important for MTP-stimulated apoB secretion, the wild type PDI subunit was replaced with an active site mutant, mPDI (Cys36 --> Ser/Cys380 --> Ser), lacking both disulfide shuffling and redox activities. MTP containing mPDI was fully functional in promoting apoB and triglyceride secretion. Therefore, the shufflase and redox activities of PDI are not necessary for the function of MTP. Since PDI exists in large molar excess over the other subunit of MTP, the role of free PDI (independent of the MTP complex) was investigated. PDI or mPDI was co-expressed with B48 and B17, a fragment encompassing the amino-terminal 17% of apoB. Mutant PDI significantly and specifically reduced the accumulation of the B17 and B48 both intracellularly and in the culture medium. The reduction was partially eliminated by the protease inhibitor N-acetyl-leucyl-leucyl-norleucinal, consistent with rapid co- or post-translational degradation of apoB in the presence of mPDI. Treating the cells with oleate reversed the effect of mPDI on B48 secretion in a dose-dependent manner, but had no effect on B17.

IN CONCLUSION

1) the role of PDI in the MTP complex involves functions other than its known enzymatic activities; 2) one or both of the enzymatic activities of free PDI is/are important for the MTP-independent steps of apoB secretion; 3) oleate can affect apoB secretion at high physiological concentrations and compensate for the insufficiency of PDI activities.

A mutant truncated protein disulfide isomerase with no chaperone activity

A mutant human protein disulfide isomerase with the COOH-terminal 51 amino acid residues deleted (abb'a') has been expressed in Escherichia coli. Its secondary structures are very similar to those of the native bovine enzyme. The mutant enzyme shows neither peptide binding ability nor chaperone activity in assisting the refolding of denatured D-glyceraldehyde-3-phosphate dehydrogenase but keeps most of the catalytic activities for reduction of insulin and isomerization of scrambled ribonuclease. It assists the reactivation of denatured and reduced proteins containing disulfide bonds, acid phospholipase A2, and lysozyme to different levels, which are significantly lower than those by the native bovine enzyme.